Telecommunications in general is advancing in many different directions. Such advancements can be divided into two categories: digital signals and analog signals. Analog signals provide conventional communication paths between two nodes in a telecommunication network. For example, a conventional home telephone may establish a connection with a central office through a pair of TIP and RING lines (collectively referred to as a telephone line) for the purpose of transmitting analog signals back and forth. The analog signals may include voice and/or analog computer modem traffic.
Digital signals also provide communication paths between two nodes in a telecommunication network. For example, a conventional home computer may establish a connection with a central office through a telephone line for the purpose of transmitting digital signals back and forth. A contemporary example would be a digital subscriber line, or DSL, for establishing a high speed data path between the computer and another node connected (directly or indirectly) to the central office.
Analog signals are defined to a predetermined bandwidth, called a voice band. The voice band typically ranges from a few hundred hertz (Hz) to about 3 kilo Hz (kHz), which coincides with a sound range for a human ear. Any signal carried on a telephone line that is in the voice band is deemed to be an "in-band" signal. Also by definition, analog signals must exceed a predetermined voltage threshold. The voltage threshold represents the amplitude of the signals, measured either in decibels (db) or volts peak-to-peak (Vp-p).
Digital signals are also defined to a predetermined bandwidth, called a data band. The data band typically ranges from 3 kHz to several thousand kHz. Any signal carried on a telephone line that is in the data band is deemed to be an "out-of-band" signal. Also by definition, data signals must exceed a predetermined voltage threshold. The voltage threshold represents the amplitude of the signals, measured either in decibels or volts peak-to-peak.
Telephone lines often require maintenance and repair. During such maintenance, a butt-in test set is often used to detect the presence of an audio signal on the line. Typically, the test set is connected across the TIP and RING lines and placed off-hook, thereby seizing the lines. If the telephone line is carrying an out-of-band digital signal, however, the person using the test set will not hear anything and inappropriately seize the line. As a result, the out-of-band digital signal will be corrupted by the test set. Specifically, the test set will provide an electrical load to the telephone line. After a period of time, this load will adversely affect the data signal, thereby corrupting the data.
Therefore, it is desired to accurately test for an out-of-band digital signal when a telephone line is to be seized. If the out-of-band digital signal is detected, it is further desired to prevent the line from being seized and to indicate such to the user.
It is also desired that the test set work properly with different types of telephone lines. For example, a central office will often supply a direct current (DC) voltage to the telephone line. This DC voltage can supply power to many conventional telephones, and has other conventional uses. However, some telephone lines do not have any applied DC voltage. It is important that a test set work with a telephone line, regardless of its type.
It is further desirable to support sequential automatic connection testing. Although the ideal method for connecting a test set to a telephone line is to first monitor the line with the test set in the on-hook mode , and then switch to the off-hook mode only after assuring that the line is not carrying data signals, it is common practice among telecom users to leave the test set in the off-hook mode and move the test set down a column of different telephone line terminals. This type of testing, called sequential automatic connection testing, can corrupt data signals in many types of telephone lines.
U.S. Pat. No. 5,617,466 describes a mechanism for controllably enabling a test set to assert off-hook condition on a telephone line if a prescribed voltage level is detected and no out-of-band digital signals are detected. However, this patent requires the existence of a DC voltage on the telephone line. Also, this patent does not support sequential automatic connection testing. Specifically, the patented circuit includes a rectifier bridge, similar to the rectifier bridge commonly found in most test sets. The additional voltage drop across the second bridge will degrade the performance of the test set in certain situations.
U.S. Pat. No. 4,939,765 describes an interlock circuit for preventing corruption of digital signals on a telephone line. However, this patent only checks for data during a limited test period. Therefore, this patent requires the existence of a DC voltage on the telephone line to perform sequential automatic connection testing. It will never know when it moves to a new telephone line that does not have a DC voltage present, and will automatically apply a data-corrupting load to the unpowered line.